The treatment is already having ‘spectacular’ effects in seriously ill melanoma patients – and could soon be used to defeat other types of cancer.

One scientist said it was ‘amazing’ that researchers could talk of ‘using the C-word – cure’ for the first time, while another said trials among kidney and lung cancer patients are ‘very exciting’.

Each year almost 13,000 people in Britain are diagnosed with melanoma, including two people aged 15 to 34 every day

One in six patients ravaged by deadly skin cancer are already being cured by the drugs, the European Cancer Congress was told yesterday, with the possibility of more than half being saved with an new combination.

The astonishing development will save the lives of thousands of patients with advanced melanoma, who usually die within months of being diagnosed. The new treatment, which will bring hope to hundreds of thousands of people in Britain with terminal conditions, involves the combination of two different types of drug that ‘reboot’ the immune system.

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The first is ipilimumab – or ‘ipi’. According to research presented to the European congress in Amsterdam yesterday, 17 per cent of patients are cured by this drug alone.

But many more – perhaps more than half – could be ‘clinically cured’ by combining them with even newer drugs called anti-PD1s which break down cancer cells’ defences.

Professor Alexander Eggermont of the Institut Gustave Roussy in France said: ‘[Advanced] melanoma could become a curable disease for perhaps more than 50 per cent of patients within five to 10 years.’

He told The Mail on Sunday: ‘If I’d made this bizarre prediction five years ago, people would have said I was mad. But it now looks like we are going to have control of advanced melanoma for years, in a substantial proportion of patients.’

Each year almost 13,000 people in Britain are diagnosed with melanoma, including two people aged 15 to 34 every day. It kills 2,200 a year, accounting for four in five skin cancer deaths. The number of cases has quadrupled in 30 years, due in part to our fondness for foreign holidays to sunny destinations.

If caught early enough, malignant moles can be cut out before the cancer spreads, but if it is missed and the cancer turns ‘metastatic’ – spreading to other organs – chances of long term survival plummet.

At that stage patients are usually told they have just months to live. Currently, less than a quarter survive a year or more.

Prof Eggermont believed these new immunotherapy drugs would be adopted ‘across the board’ to successfully treat a wide range of other advanced ‘solid tumour’ cancers.

He said: ‘These drugs are going to help a large number of people. It’s a massive deal. Reports from trials with patients who have advanced kidney and lung cancer are very exciting.’

In one lung cancer trial using anti-PD1 drugs, tumours markedly shrank in a quarter of very ill patients who had failed to respond to chemotherapy, he said. Trials had also been started in patients with prostate, bowel and breast cancer.

Prof Stephen Hodi, of the Dana-Farber Cancer Institute in Boston, found encouraging results in the survival chances of almost 5,000 melanoma patients put on ipi. ‘This is a really amazing time,’ he said. ‘A few years ago we could never have imagined using the C-word – cure – in melanoma. But we are headed that way.’

Cancer scientists have tried for decades to create drugs which help the immune system fight cancer, but until very recently immunotherapy appeared to be a dead-end.

Prof Eggermont said 24 of his 27 years in the field had been ‘very difficult . . . I couldn’t see the light at the end of the tunnel.’

Most attempts had failed because they had concentrated on the wrong thing: activating cancer-fighting T-cells, rather than breaking down the tumours’ defences.

He said: ‘All these attempts ended when they came up against the wall of defence. We needed to break down that wall and open up the tumour to attack, which ipi and the anti-PD1s do in different ways. Now it’s a totally different ball game.’

British experts yesterday said there was ‘enormous excitement’ at the new drugs – although they stressed it was still early days.

Professor Peter Johnson, chief clinician at Cancer Research UK, said: ‘We have always thought that if you can get the immune system to respond to a tumour, then it’s likely to be a long-lasting effect, because the immune system is designed to last for life. So it’s very exciting that we are starting to see it working.’

Dr James Larkin, of the Royal Marsden Hospital in London, said ipi alone was ‘a vast improvement’ on previous treatment. ‘Traditionally, when melanoma has spread to other organs, it is regarded as incurable. So this is big news.

‘It’s difficult to know that we have actually ‘cured’ people, because we have to wait to see if they die from something else. But all this new ipi data points to the fact that if you respond to the drug and are still alive three years after treatment, then it’s very likely that you are clinically cured. It’s a very big story.’ The breakthrough comes after David Cameron announced the extension of a £200 million-a-year budget to pay for cancer treatments denied to NHS patients because rationing body NICE believes they are poor value for money.

Ipi is already available on the NHS, under the brand name Yervoy, although its course of four treatments is very expensive.

Dr Larkin is now working on a trial to combine ipi with anti-PD1 and said early results have shown ‘spectacular levels of tumour shrinkage’.

Initial trial results presented earlier this year showed that tumours shrank by more than half in 11 per cent of patients put on ipi, in 41 per cent put on anti-PD1s – but in 53 per cent of those given both.

Professor Paul Workman of the Institute of Cancer Research described the immune-boosters as ‘a real breakthrough’, adding: ‘I think we will have the expertise to take more patients to five, ten years [of life] and eventually cure them.’

Charlotte Fionda of the Karen Clifford Skin Cancer Charity said: ‘It is really encouraging to see positive data. We hope that this is just the beginning.’